miRNAs Signature in Head and Neck Squamous Cell Carcinoma Metastasis: A Literature Review

Statement of the Problem Head and neck cancers include epithelial tumors arising in the oral cavity, pharynx, larynx, paranasal sinuses, and nasal cavity. Metastasis is a hallmark of cancer. MicroRNAs (miRNAs) are endogenous small non-coding RNAs involved in cell proliferation, development, differentiation and metastasis. It is believed that miRNA alterations correlate with initiation and progression of cancer cell proliferation or inhibition of tumorigenesis. Moreover, miRNAs have different roles in development, progression, and metastasis of head and neck squamous cell carcinoma (HNSCC). Altered expression of miRNAs could be novel molecular biomarkers for the definite diagnosis of cancer, metastatic site, cancer stage, and its progression. Purpose The purpose of this review was to provide a comprehensive literature review of the role of miRNAs in head and neck cancer metastasis. Search strategy A relevant English literature search in PubMed, ScienceDirect, and Google Scholar was performed. The keywords ‘miRNA’, ‘head and neck’, and ‘cancer’ were searched in title and abstract of publications; limited from 1990 to 2015. The inclusion criterion was the role of miRNAs in cancer metastasis. The exclusion criterion was the other functions of miRNAs in cancers. Out of 15221 articles, the full texts of 442 articles were retrieved and only 133 articles met the inclusion criteria. Conclusion Despite the advances in cancer treatment, the mortality rate of HNSCC is still high. The potential application of miRNAs for cancer therapy has been demonstrated in many studies; miRNAs function as either tumor suppressor or oncogene. The recognition of metastamir and their targets may lead to better understanding of HNSCC oncogenesis, and consequently, development of new therapeutic strategies which is a necessity in cancer treatment. Development of therapeutic agents based on miRNAs is a promising target.


Introduction
Head and neck cancer is the sixth most common malignancy, comprising 6% of all cancers [1] and includes epithelial tumors arising in the oral cavity, pharynx, larynx, paranasal sinuses, and nasal cavity. Almost all of these malignancies are squamous cell carcinoma (SCC). [2] Head and neck squamous cell carcinomas (HNSCCs) are highly heterogeneous cancers. Oral squamous cell carcinoma (OSCC) is the most frequently occurring cancer in the head and neck area (90% of all cases). Metastasis is the main cause of death [3] and mostly arises in the tongue, floor of mouth, gingiva, and buccal mucosa. [4] Metastasis is a hallmark of cancer and is defined as the transfer of disease from one organ or part of an organ to another part that is not directly connected to it.
Tumor cells penetrate into vascular or lymphatic channels and provide the opportunity to spread. The spread is dictated by local anatomy, and each site has its own pattern. [5] The process of metastasis is complex and involves sequential steps. First, the cells detach from the primary site. Then, they spread in the tissue, move away through the extracellular matrix, invade blood vessels, and settle in the microvasculature. Finally, the cells extravasate through the vessel wall and proliferate in the recipient tissue. [6] Cervical lymph node metastasis is the strongest determinant of patient prognosis in HNSCCs [7] which decreases the survival rate by about 50%. [8][9] HNSCC is not usually detected in the early stages of the disease due to the absence of clinical symptoms. [10] MicroRNAs (miRNAs) are endogenous small non-coding RNAs; they can regulate gene expression in the post-transcriptional stage by interacting with the 3' untranslated region (3' UTR) of the target mRNA. [11] Several miRNAs regulate cancers; miRNAs play crucial roles in proliferation, differentiation, apoptosis, survival, motility, invasion and metastasis, and morphogenesis.
[12] It has been shown miRNAs can be used as novel molecular biomarkers for cancer diagnosis. [13] For example, circulating miRNAs are associated with distinct metastatic site; therefore, they are powerful tools to evaluate the disease stage and progression. [14] The purpose of this review was to provide a comprehensive literature review of the role of miRNAs in head and neck cancer metastasis. The role of miRNAs in regulating EMT/MET Invasion is the initial step in the metastatic process. The mode of tumor invasion is the most significant prognostic factor for the presence of lymph node metastasis.  promotes cancer cell proliferation. [40] A previous study on OSCC cells indicated that transfecting with miR-125b or miR-100 significantly decreased cell proliferation; however, co-transfection had a greater effect on proliferation than individual transfection. [41] miR-128 also acts as a tumor suppressor and inhibits the growth of HNSCC cells. [42] Down regulation of miR-29 in OSCC [43] is a positive regulator of p53. [44] miR-15 and miR-16 are the regulators of Bcl-2, an anti-apoptotic factor. Therefore, endogenous levels of miR-15 and miR-16 correlate inversely with the Bcl-2 protein level. [45] In an in vitro study, miR-375 induced apoptosis in HNSCC by targeting TNF-α. [46] The role of miRNAs in cancer development and metastasis Previous studies have indicated that dysregulation of miRNAs plays a crucial role in the progression of oral precancerous lesion from dysplasia to OSCC. For instance, miR-31 negatively controls oral leukoplakia progression through the regulation of fibroblast growth factor 3 (FGF3). On the contrary, miR-21, miR-181b, and miR-345 are up-regulated in oral dysplasia and associated with lesion severity. [47] Overexpression or subexpression of miRNAs in cancer has the ability to activate or block the target mRNAs. In a metastatic carcinoma, the miRNA expression profile has been shown to be different from a non-metastatic tumor. [48] Therefore, interference with the expression level of miRNAs has an effect on cancer prognosis. This finding provides a therapeutic use for miRNAs. [49] Regarding the role of miRNAs in cancer, they are divided into two main groups of tumor suppressor and oncogenes (oncomirs). Altered miRNA expression depends on its role as a tumor suppressor or oncogene.

Tumor suppressor miRNAs involved in HNSCC metastasis
The literature review shows miR-1 is down-regulated in hypopharyngeal SCC. It has been shown miR-1 can suppress metastasis in HNSCC by targeting TAGLN2, a gene which mediates cell migration and invasion. [53] Let-7d, a member of the let-7 family of miRNAs acts as a tumor suppressor, most likely via targeting RAS. [54] A reduction of let-7d expression in OSCC tissues has been reported. Let-7d was negatively correlated to Twist, Snail, and EMT transcription markers in OSCC cell lines. In addition, let-7d was significantly decreased in regional metastatic lymph nodes of OSCC patients.
[55] Combined low levels of miR-205, and let-7d expression are associated with distant metastasis. [56] Moreover, decreased let-7d expression in HNSCC is associated with poor prognosis. Both miR-17 and miR-20a are down-regulated in advanced migratory OSCC, and their expression levels are negatively controlled by advanced TNM stage and lymph node metastasis.
ITGβ8 is a direct target of both miR-17 and miR-20a.
Thus, they can be used as prognostic biomarkers for OSCC. [57] The miR-29 family is significantly downregulated in HNSCC tissues and cell lines suggesting their contribution to metastasis in HNSCC through laminin γ2 (LAMC2) and α6 integrin (ITGA6). [58] A previous study demonstrated that the transforming growth factor beta (TGF-β) signaling which contributes to EMT process, [59]

OncomiRs involved in HNSCC metastasis
It is reported that miR-10b is down-regulated in HNSCC cell lines and induces cell proliferation. [79] miR-10b significantly increases oral cancer cell migration and invasion. [80] Recently, miR-21 has been studied more frequently as a prognostic marker of HNSCC.
It is stated that miR-21 down-regulates programmed cell   Table   2 summarizes the role of oncomiRs in HNSCC metastasis. [96] Besides, the different expression profile of miR-NAs in esophageal squamous cell carcinoma is a diagnostic and prognostic factor as well as a tool for distinguishing cancer from normal tissue. [97] Moreover, miRNAs help to predict the primary site of metastatic disease. [11] In head and neck area, the primary site is unknown in up to 10% of metastatic squamous cell carcinomas. By knowing the origin of the disease, specific therapeutic regimens such as reducing the radiation field can be employed, which consequently minimizes the morbidity.

Discussion
Due to a gradual increase in the number of cancer patients, careful evaluation of patients has a great impact on decreasing the cancer-related morbidity and mortality rate. Among the cancer groups, HNSCCs consist of highly heterogeneous lesions. [103] It seems that in the metastatic phenotype of miR NAs, a global reduction of miRNA abundance plays a causal role; [11] however; several miRNAs are upregulated in specific tumors. Moreover, miRNAs act as either oncogenes or tumor suppressors which make them prospective targets for therapeutic development against cancers. [104] There are many challenges in the application of miRNA-based therapies.
Similarly, miR-1 down-regulation was observed in other types of cancer such as lung cancer. [105] A previous study indicated that miR-29 was downregulated in breast cancer; miR-29 has an essential role in inhibiting growth of breast cancer cells via downregulation of B-Myb. [106] But in HNSCC tissue, the target genes of mir-29 are LAMC2 and ITGA6. [58] The miR-34 family may inhibit invasion and metastasis in different cancer types. For example, miR-34a and b are suggested as tumor suppressors in several cancers such as gastric cancer, [107] breast cancer, [108] and hepatocellular carcinoma mainly by targeting p53. [109] mir34a/c may function as a metastasis suppressor in breast cancer through targeting Fos-related antigen 1 (FOSL1). [110] Nonetheless, the target gene of miR-34 in HNSCC is not clear. Twist which induces EMT is a target of miR-10b. [114] Moreover, miR-10b targets E-cadherin and modulates metastasis in breast cancer; [115] however; its target gene in HNSCC is still unknown. The literature shows that miR-21 is one of the most studied miRNAs. [116] Although miR-21 functions as an oncogene via programmed cell death 4(PDCD4) in HNSCC, [81] and esophageal carcinoma, [117] it stimulates cell invasion and promotes metastasis in breast cancer by targeting both PDCD4, and tropomyosin 1 (TPM1), a tumor suppressor gene. [118][119] On the other hand, miR-21 controls esophageal squamous cell carcinoma invasion by targeting the tissue inhibitor of matrix metalloproteinase-3 (TIMP3) molecules which control ECM. [120] miRNAs expression pattern and function are cancertype-specific. For example, up-regulation of miR-31 has been reported in colorectal cancer; [121] however, reduced level of miR-31 was observed in breast cancer. [122] While it inhibits metastasis in breast cancer, [123] it induces invasion and metastasis in colon cancer via TGF-β. [124] In addition, miR-181a and mir-181b suppress glioma tumor growth, [125] but contributes to tumor progression in thyroid papillary carcinoma. [126] miR-211 acts as a tumor suppressor in melanoma, [127] whereas; it functions as an oncomir in colon cancer. miR-363 is a tumor suppressor in T cell lymphoma, [130] however; it acts as an oncogene in breast cancer. [131]

Conclusion
In conclusion, despite advances in cancer treatment, the mortality rate of HNSCC is still high. The potential ap-